This application is based upon and claims benefit of priority of Japanese Patent Applications No. Hei-10-288185 filed on Oct. 9, 1998 and No. Hei-11-245939, filed on Aug. 31, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an electroluminescent panel that has a luminescent layer made of organic chemical compounds sandwiched between a pair of electrodes and emits multi-color light.
2. Description of Related Art
An example of organic electroluminescent panels emitting multi-color light is disclosed in JP-A-8-78163. This panel includes a hole transport luminescent layer and an electron transport luminescent layer, both luminescent layers being laminated with a carrier re-combination control layer interposed therebetween. Both luminescent layers simultaneously emit respective colored lights that are combined into a white light when they are emitted from the panel. The hole transport luminescent layer is composed of a hole transport material as a host material and a fluorescent material as a dopant. Similarly, the electron transport luminescent layer is composed of an electron transport material as a host material and a fluorescent material as a dopant. Both the host and dopant materials show fluorescence property in their solid state.
In this electroluminescent panel, positive holes are injected into the hole transport luminescent layer and electrons are injected into the electron transport luminescent layer from the pair of electrodes composed of an anode and a cathode. The injected holes and electrons re-combine in the luminescent layers through the carrier re-combination control layer, thereby generating excitons. The excitons transfers energy to the host materials and/or fluorescent dopants, and thereby light is emitted from the luminescent layers. The region where the carrier re-combination occurs varies according to the thickness of the carrier re-combination control layer interposed between both luminescent layers. That is, the carriers may re-combine in either one of the luminescent layers, or in both layers, according to the thickness of the control layer. Thus, light of desired color. can be emitted from the luminescent panel by properly selecting the thickness of the control layer. It is confirmed that white light or arbitrary colored light can be emitted from the panel disclosed in the foregoing publication by properly controlling the thickness of the control layer and by mixing the different colored lights simultaneously emitted from the respective luminescent layers.
However, it is difficult to properly control the thickness of the carrier re-combination control layer in the manufacturing process, because it is considerably thin, e.g., 3 nm thick. If the thickness deviates from the target, desired colored light cannot be obtained. For example, when the thickness is too thin, only the electron transport luminescent layer emits light. On the other hand, when it is too thick, only the hole transport luminescent layer emits light. Therefore, white light resulting from mixing both colored lights may not be obtained even if it is desired.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an organic electroluminescent panel that is able to emit white light or arbitrary colored light without interposing the carrier re-combination control layer between the hole transport and the electron transport luminescent layers.
A hole transport luminescent layer and an electron transport luminescent layer are laminated so that both layers directly contact each other. Both luminescent layers are sandwiched between an anode and a cathode. The hole transport luminescent layer is positioned at a side of the anode, and the electron transport luminescent layer is positioned at a side of the cathode. The hole transport luminescent layer is composed of a hole transport host material and a first fluorescent material doped in the host material. Similarly the electron transport luminescent layer is composed of an electron transport host material and a second fluorescent material doped in the host material. Both luminescent layers simultaneously emit respective lights when a voltage is imposed on the pair of electrode layers. The lights emitted from both luminescent layers are mixed into a white light or any other colored light. The color of the mixed light is arbitrarily controlled by selecting combination of the host materials and the dopants.
In order to simultaneously emit light from both luminescent layers, combination of materials has to be properly chosen. If the material combination is improper, only one luminescent layer emits light while the other luminescent layer does not. For example, this occurs in the following material combination. The host material of the hole transport luminescent layer: xcex1-naphthylphenylbenzen (xcex1-NPD, emitting blue light), the first fluorescent material: perylene (emitting blue light), the host material of the electron transport luminescent layer: tris(8-quinolyl)aluminum (Alq, emitting green light), and the second fluorescent material: 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM1, emitting red light). In this example, DCM1 emits light but perylene does not, and the panel as a whole emits orange light.
This undesirable phenomenon occurs mostly for the following reasons. First, it is difficult for the carries (holes and electrons) to cross the boundary of both luminescent layers, because the energy levels (the minimum energy level in the conduction band or the maximum energy level in the valence band) are different to a large extent between both host materials. Accordingly, the chance of re-combination of the carries in both luminescent layers is small. In the particular example given above, holes can move from the hole transport luminescent layer to the electron transport luminescent layer, but movement of electrons in the reverse direction is difficult. Secondly, there is an energy gap difference between xcex1-NPD (host material of the hole transport luminescent layer) and Alq (host material of the electron transport luminescent layer). The energy gap that is a difference between the conduction band minimum energy level and the valence band maximum energy level is peculiar to each material, and the energy gap of a material having a shorter luminescent wavelength is higher. The energy gaps of xcex1-NPD and perylene are higher than that of Alq, while the energy gap of Alq is higher than that of DCM1. Therefore, the exciton energy generated in the electron transport luminescent layer does not move to the hole transport luminescent layer across the boundary, while it moves to the dopant DCM1. The exciton energy generated in the hole transport luminescent layer easily moves to the electron transport layer across the boundary.
Based on the consideration mentioned above, both host materials of the luminescent layers are selected so that their energy level difference becomes small and carriers can easily move across the boundary, and the combination of the host materials and the dopants is chosen so that the exciton energy becomes high and it moves beyond the boundary. More particularly, both host materials are selected so that their wavelength showing a solid state fluorescence spectrum peak commonly falls within a range of 380 nm-510 nm. It is preferable to use a material, the valence band maximum energy level of which is xe2x88x925.6 eV or higher, as the host material of the hole transport luminescent layer, and to use a material, conduction band minimum energy level of which is xe2x88x922.7 eV or lower, as the host material of the electron transport luminescent layer. The first and the second fluorescent materials to be doped in the host materials are selected so that their solid state fluorescence spectrum overlaps with that of the host materials or is shifted to the longer wavelength side. By selecting the materials as above, carriers can easily move across the boundary of both luminescent layers, and the fluorescent materials to be doped in the host materials can be chosen from a whole range of visible light. Both luminescent layers simultaneously emit respective lights that are mixed into a white light or any colored light desired. Further, electric energy is efficiently converted into light energy.
To further alleviate the energy barrier of the boundary of both luminescent layers, the fluorescent materials doped in the luminescent layers are selected, so that the conduction band minimum energy level of one of the fluorescent materials falls within a range between the conduction band minimum energy levels of both host materials, and the valence band maximum energy level of the other fluorescent material falls within a range between the valence band maximum energy levels of both host materials. The energy barrier of the boundary is determined by the conduction band minimum energy level difference (referred to as Dc) between both luminescent layers and the valence band maximum energy level difference (referred to a Dv) between both luminescent layers. Though those energy level differences are mostly determined by the host materials, they can be decreased by doping the fluorescent materials having energy levels as recited above. Thus, the carriers and excitons can move more easily across the boundary.
It is also preferable to make the difference between Dc and Dv smaller to further promote the movement of carriers, holes and electrons, in both direction. In the above example in which xcex1-NPD is used as the host material of the hole transport luminescent layer and Alq as the host material of the electron transport luminescent layer, only holes are injected from the hole transport luminescent layer to the electron transport luminescent layer but the electrons do not move in the reverse direction. This undesirable phenomenon is caused for the following reasons. The conduction band minimum energy level of xcex1-NPD is xe2x88x922.4 eV and that of Alq is xe2x88x923.0 eV, and accordingly Dc is 0.6 eV. The valence band maximum energy level of xcex1-NPD is xe2x88x925.4 eV and that of Alq is xe2x88x925.7 eV, and accordingly Dv is 0.3 eV. Since Dc is larger than Dv in this example, only holes can be injected while electrons cannot.
It is found out that the carriers move in both directions if the difference between Dc and Dv is 0.2 eV or less. If Dc and Dv is substantially balanced, movement of electrons and holes are properly blocked by the boundary, and their re-combination occurs in both luminescent layers at the vicinity of the boundary. Accordingly, high energy conversion efficiency is realized.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment and examples described below with reference to the following drawings.